Numerous magnesium alloys having a wide variety of compositions and uses are known. For purposes of the present disclosure, the term “magnesium alloy” is understood to refer to a group of alloys, which include, but are not limited to, in addition to magnesium as the main constituent, additives (usually up to approximately 10%) of aluminum, manganese, zinc, copper, nickel, cerium mixed metal and other rare earth metals, silver, zirconium, silicon, combinations thereof and the like. Magnesium alloys are divided into Mg wrought alloys (usually, but not exclusively, based on Mg—Mn, Mg—Al—Zn) and Mg cast alloys. The MG cast alloys are divided into sand casting, chill casting and die casting according to the alloy constituents. Magnesium alloys can be processed by most known primary forming methods and reshaping methods.
The alloy additives determine the properties of the metallic material to a significant extent. For example, it is known that an aluminum content of more than approximately 10 weight-percent leads to embrittlement of the alloys. Zinc, and especially zirconium, increases the toughness, whereas manganese improves the corrosion resistance. Beryllium additives in the amount of a few ppm will significantly reduce the oxidation tendency of the molten metal but beryllium additives are undesirable because of their toxicity. Rare earth metals and thorium increase the high-temperature strength. The melting point of the alloys is usually between 590° C. and 650° C.
The main areas of use of magnesium alloys include aviation, mechanical engineering of all types, optical equipment, electrical engineering, electronics, conveyance means, office machines and household appliances, and in areas where strength and rigidity with the lowest possible weight are important, while also achieving low manufacturing costs in large series. Magnesium alloys are becoming increasingly important in automotive engine construction. A special application relates to the use of biodegradable magnesium alloys in medical technology, in particular, for vascular and orthopedic implants.
One limit of known magnesium alloys is the ductility of the material, which is often inadequate for certain processing methods and intended purposes. One approach to improve ductility might be to reduce the grain size of the metallic structure (refining). Refining includes all metallurgical measures that lead to a small grain size of an alloy. In general, this presupposes increasing the seed count in the melt in solidification or in the solid state by finely dispersed precipitates. Refining has an advantageous effect on the mechanical properties, in particular, the ductility of the alloy. In the field of magnesium alloys, very small grain sizes have previously been achieved only on a small scale preparatively, e.g., by ECAP/ECAE methods (ECAP stands for equal channel angular pressing; ECAE stands for equal channel angular extrusion). However, the aforementioned methods cannot be implemented on a large scale industrially; so far only small volumes (a few cm3) of extremely fine-grained alloys have been produced, primarily with the industrial magnesium alloy AZ31. So far, there has been only inadequate information of a generally valid nature about the alloy components required for refining or even their amounts in magnesium alloys.
There is, therefore, an ongoing demand for magnesium alloys that allow refining even with access to traditional large-scale industrial methods. In addition, there is a demand for a magnesium alloy having a reduced grain size in comparison with traditional alloys with regard to improved ductility. Furthermore, there is a demand for a production method for a fine-grained magnesium alloy that can be implemented technically on a large scale. Finally, from the standpoint of ecological aspects as well as technical medical use of the alloys, it is necessary to select the alloy components based on toxicological and/or biocompatible factors; biocompatible factors are important, in particular, to avoid the aluminum that is present in many magnesium alloys.